Vessel and unloading system

ABSTRACT

A system and method for transferring fluid from a vessel to a shore-based facility, the system including at least one vessel and at least one coupling structure which is established at an unloading location. The vessels travel in continuous shuttle traffic between a loading station and the coupling structure and are organised such that at least one of the vessels is periodically connected to one of the coupling structures so that the fluid is conveyed from each vessel through the coupling structure and directly ashore via a pipeline system for further distribution of the fluid. Vessels are provided with storage tanks for storing natural gas in its liquid state, a vaporizer for regasifying the liquefied natural gas on arrival at the unloading point and equipment for connection to the coupling structure for transferring natural gas from the vessel to a submerged pipeline system.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a special vessel together with a systemand method for unloading fluid from the vessel to a shore-basedinfrastructure.

[0002] In connection with the transport of fluid such as natural gasfrom offshore wells to the area in which the natural gas has to beunloaded, a system is often employed involving a number of specialvessels and a series of fairly complicated operational steps. A knownvessel which is commonly employed for this purpose is an LNG carrier,which is equipped with special tanks suitable for storing natural gas inits liquid state. When loading on board the LNG carrier in the field, atleast one additional vessel is often employed for receiving fluiddirectly from the wells and treating the fluid before it is transferredto the LNG carrier. According to common practice, on arrival at theunloading point the LNG carrier will transfer the load to anintermediate storage unit where the liquefied natural gas is convertedto a gaseous state before being transferred to the end user.

[0003] These systems, requiring the use of several different specialvessels and the performance of related complicated operations, are shownto be complex and extremely costly.

[0004] The following special vessels and systems are known from thepatent literature:

[0005] In U.S. Pat. No. 6,089,022 an unloading system is describedinvolving an LNG carrier which is equipped with spherical tanks forstoring liquefied natural gas (LNG) and vaporizers for regasifying theliquefied natural gas. On arrival at the unloading point, the carrier ismoored in such a manner that the vessel is located at a distance fromthe mooring structure by mooring lines which extend from the bow area tothe mooring structure. The liquefied natural gas is regasified beforebeing transported in a pipeline system which transfers the natural gasfrom the carrier to a shore-based installation which in the publicationrepresents the end user.

[0006] In the arrangement disclosed in U.S. Pat. No. 6,089,022, a singlepipeline is employed for unloading the natural gas from the vessel toshore. In FIG. 1 of that patent, this pipeline is illustrated arrangedin such a manner that it extends from the bow area, through the mooringstructure, from where there is provided yet another pipeline whichtransfers the natural gas to shore. In the publication there is noindication of any possibility of using a buoy structure or alternativetechnical solutions, which can be mounted in the vessel's hull forunloading natural gas or a transfer structure or alternative technicalsolutions which can be attached to a structure projecting from thevessel's bow area.

[0007] U.S. Pat. No. 5,564,957 discloses a vessel in which a buoystructure is provided for installation in a recess in the bow portion.The buoy structure is intended for use as a combined mooring andtransfer structure for transport of a liquid medium to and from thevessel.

[0008] In U.S. Pat. No. 6,094,937, a processing plant is described forconverting natural gas to a liquid state (LNG) and a shuttle system fortransporting LNG from the field. In this shuttle system, two buoys andfour carriers may be employed for transporting LNG in order to maintainalmost continuous production in the field.

[0009] The existing technical solutions disclosed in these knownpublications demonstrate an inadequacy in relation to the complexrequirements, which is eliminated or reduced by the present invention.

BRIEF SUMMARY OF THE INVENTION

[0010] It is an object of the present invention to reduce the number ofspecial vessels which must be included in such a system, and to improvethe characteristics of these special vessels in relation to theoperations which have to be carried out.

[0011] A more specific object of the present invention to provide asystem for unloading fluid, and especially natural gas, wherein thecarrier vessel has storage tanks as well as a vaporizer on board, and isarranged to receive a coupling structure, which may be a buoy structurewhich has both a mooring function and a transfer function, or a transferstructure that can be connected to a receiving structure that projectsfrom the vessel's bow area.

[0012] It is a further object of the present invention that the vesselwith its special functions should be able to form part of a shuttlesystem which in a preferred embodiment includes a plurality of vesselsand a plurality of the couipling structures concerned. The object ofthis system is to achieve the continuous supply of natural gas from theunloading point to the infrastructure.

[0013] The technical solution according to the present invention makesuse of a coupling structure, which may be a submerged buoy structurewhich has to be brought into abutment in a recess in the vessel's hull,or a transfer structure which has to be brought into abutment with aportion projecting from the vessel's bow, for transferring natural gasto shore. In contrast to the technique disclosed in U.S. Pat. No.6,089.022, the coupling structure offers a substantially simplifiedsolution which provides advantages both with regard to manning and theequipment situation, since mooring and fluid transfer are implemented bymeans of one and the same structure.

[0014] The technique disclosed in U.S. Pat. No. 5,564,957 differs fromthe invention in that there is no vaporizer provided on board the vesseldisclosed in that patent, nor is there any indication in that patentthat the medium has to be transferred directly to a shore-basedinstallation.

[0015] A basic difference between the present invention and the systemdisclosed in U.S. Pat. No. 6,094,937 is that the system disclosed in thepatent is used in connection with loading, while the system according tothe invention has to be used for unloading. This is manifested by thefact that a liquefier is provided on board the carrier in U.S. Pat. No.6,094,937, while a vaporizer is provided in the LNG tanker according tothe invention. In U.S. Pat. No. 6,094,937 there is no suggestion thatthis loading/transport system will be capable of use in connection withunloading. Nor will a vessel like that disclosed in U.S. Pat. No.6,094,937 be able to be used for unloading natural gas to a shore-basedinfrastructure without the need for considerable structuralmodifications to the vessel.

[0016] With the vessel according to the invention, a number of theoperations required when using the previously known systems are madesuperfluous. Specifically, a number of the operations associated withmooring, connection of pipelines, transfer of liquid cargo from onevessel to another will be made superfluous and/or simplified. Inaddition, when using the system according to the invention the number ofcrew members needed will be reduced, and thereby also the operatingcosts, due to the fact that both the number of operations performedbecomes less and more functions are concentrated on one vessel.

[0017] The design and production of a multi-functional vessel of thiskind can be an extremely costly and time-consuming process. It istherefore another object of the present invention to enable the vesselto be built in a cost-effective manner, thus enabling the vessel to beoffered at a competitive price.

[0018] In preferred embodiments of the invention the vessel is providedin a simple and inexpensive manner using a standard LNG carrier as thebasis. The carrier with its storage tanks for liquefied natural gas isequipped with one or more vaporizers preferably in the deck area.According to one preferred embodiment of the invention, the vessel has adownwardly opening receiving recess formed in the hull for receiving thebuoy structure. According to a second preferred embodiment of theinvention, a receiving device is built in the bow area for receiving thetransfer structure. Both embodiments make the vessel suitable forperforming several types of operation. This special vessel is describedas a “Shuttle and Regas Vessel (SRV)”, and the vessel may of course alsobe used as an ordinary LNG carrier.

[0019] In preferred embodiments, the vessel according to the inventionis equipped with spherical tanks for storing liquefied natural gas, butother types of storage tanks may also be suitable, such as, for example,membrane tanks. These different types of storage tanks are well-knownfrom the patent literature and are in practical use. A person skilled inthe art will be able to select the type of tank which is most suitablein an individual case.

[0020] The principles and equipment to be used in connection with theregasification of the liquefied natural gas also represent known per setechnology. Sea water may be utilized as a heat exchange medium in thevaporization process, but also other media such as, for example, propaneeither alone or together with sea water, and a water-glycol mixture maybe suitable vaporization media.

[0021] The buoy structure which is to form part of the system may bedesigned in many ways. From the patent literature, including patentscited above, several examples are known of submerged buoy structureswhich can be connected to a vessel, thus enabling the vessel to rotatefreely around the buoy. However, with regard to the buoy structure'smode of operation, rapid connection and disconnection of the buoy are arequirement in order that the unloading of fluid can start almostimmediately without delays due to time-consuming mooring procedures.

[0022] When the buoy structure is not in use, it is in a submergedcondition. On arrival at the unloading area, the vessel will pick up anauxiliary buoy, which has a line attached to the buoy structure. Theline is used to convey the buoy structure to the surface, whereupon thebuoy structure is brought into abutment in the vessel's recess. Suchprocedures, and devices for their implementation are already known inthe art.

[0023] Around its circumference the buoy structure is equipped withmooring lines which extend down to mooring points on the seabed Risersare provided up to the middle of the buoy structure and a swivelstructure is arranged above the buoy structure. By means of thisarrangement fluid can be passed through the buoy structure via the riserto the submerged pipelines, while at the same time the vessel rotatesaround the buoy structure. Arrangements of this type are also alreadyknown in the art. The submerged pipelines transport fluid away from thevessel towards the shore-based infrastructure.

[0024] The transfer structure is attached to one end of a riser, whilethe other end of the riser is equipped with a coupling which providesfor the connection of the riser to a submerged pipeline system. Thetransfer structure attaches the riser to the vessel's projecting bowportion, thus enabling natural gas to be transferred from the vessel tothe on-shore infrastructure. Included in the transfer structure is arapid coupling device which induces a rotating movement (a swivel) andcan be attached to the end of the riser or to the vessel, as well as thenecessary valves such as an emergency connection valve, a non-returnvalve, a shut-off valve and a flow meter.

[0025] When the transfer structure with the riser are not in use, theyare in a submerged condition. On arrival at the unloading area, thevessel will use a dynamic positioning system (DP) to manoeuvre itselfinto position to pick up an auxiliary buoy floating on the surface. Tothe auxiliary buoy is attached a line which in turn is connected to thesubmerged transfer structure. The line is hauled up, thereby pulling thetransfer structure to the surface, whereupon the transfer structure isbrought into abutment against the bow portion's projecting portion.Liquefied natural gas will be passed from the storage tanks to thevaporizer where the fluid is regasified and passed through the transferstructure to submerged pipelines which convey the natural gas to ashore-based infrastructure. When the unloading from the vessel iscompleted, the transfer structure will be released from the projectingbow portion and returned to its submerged state. In the submerged statethe transfer structure with the riser will either be located on theseabed or floating at a level some distance below the surface of thesea.

[0026] The vessel may employ dynamic positioning to maintain positionboth at the unloading point, as mentioned in the paragraph above, andduring the actual unloading operation. The dynamic positioning isundertaken on the basis of signals received from a transducer which islocated on the seabed and/or from signals from DGPS.

[0027] The invention provides systems for achieving an efficientunloading of fluid to a shore-based infrastructure.

[0028] According to preferred embodiments of the invention, two couplingstructures and two or more carrier vessels are included in the system.The number of vessels included in the system depends on the distance tothe loading point. One and the same vessel will alternate betweendifferent operations such as loading at a loading station, for examplein the field, transporting LNG from the loading station to the unloadingpoint and regasifying liquefied natural gas with subsequent transfer toa receiving system on shore. When the unloading of fluid through thecoupling structure takes place almost continuously by having at leastone vessel connected to one of the coupling structures at all times,while the other vessels are on the way to or from the loading stations,optimal efficiency is achieved in the utilization of the system.

[0029] On arrival at the unloading point, a coupling structure will beinserted in the receiving recess in the carrier vessel. Liquefiednatural gas will be passed from the storage tanks to the vaporizer wherethe fluid is regasified and passed directly through the couplingstructure to submerged pipelines which convey the natural gas to ashore-based infrastructure.

[0030] The shore-based infrastructure may take many forms, as long asthe infrastructure is suitable for receiving the regasified fluid whichis sent ashore from the carrier and the fluid is further distributed tothe end users. For example, the infrastructure may comprise a pipelinenetwork which conveys the natural gas directly to the end user or theinfrastructure may comprise a depot which is connected to appropriatetransport means for further transport of natural gas to the consumers,etc.

BRIEF DESCRIPTION OF THE DRAWING

[0031] The invention will now be explained in more detail with referenceto the figures in which:

[0032]FIG. 1 is a side view of a carrier vessel with the necessaryequipment according to a first embodiment of the invention.

[0033]FIG. 2 is a perspective view of one of the vessels shown in FIG. 1coupled up to one of several buoy structures and of the connection ofthe pipelines with the infrastructure.

[0034]FIG. 3 is a general view of the organization of the vessels shownin FIG. 1 in order to obtain an efficient unloading of fluid.

[0035]FIG. 4 illustrates two vessels as shown in FIG. 1 connected to twobuoy structures simultaneously.

[0036]FIG. 5 is a view similar to that of FIG. 1 of a carrier vesselwith the necessary equipment according to a second embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The invention will be described first with reference to anembodiment in which the vessel is arranged to be coupled to a buoysystem. As will become apparent from the description, the illustrationsprovided in FIGS. 2 and 4 are applicable, with some modification, to thesecond embodiment shown in FIG. 5. The illustration provided in FIG. 3is directly applicable to the embodiment of FIG. 5.

[0038]FIG. 1 is a simplified schematic view of a retrofitted LNG carrier“Shuttle and Regas Vessel (RSV)” according to the invention. The vessel1 is illustrated provided with a plurality of tanks 2 for storingliquefied natural gas. The figure shows how in the vessel's hull,generally along the keel, there is provided a downwardly opening conicalrecess 5 which forms a receiving arrangement for the buoy structure 7(not illustrated in FIG. 1). The vessel 1 is further provided with atleast one vaporizer 4. In order to improve the vessel's maneuverability,the vessel is also equipped with thrusters 6 in the vessel's bow portionand stern portion, respectively.

[0039]FIG. 2 illustrates an arrangement of buoy structures 7 which areconnected to a submerged pipeline system. The submerged pipeline systemcomprises risers 8 each attached to a respective one of the buoystructures 7, and pipelines 9 each connected to the end of a respectiveone of the risers 8. The pipelines 9 are connected to an on-shoreinfrastructure, illustrated here in the figure by an underwater pipelinesystem 10 and a shore-based pipeline system 12.

[0040] The buoy structures 7 are equipped with mooring lines 11extending from mooring points on the seabed to attachment points on thecircumference of the buoy structure.

[0041]FIG. 2 illustrates two vessels 1, 1′ where one of the vessels 1 isconnected to one of the buoy structures 7, while the other vessel 1′ isenroute to or from the loading point. On connection of vessel 1 with thebuoy structure 7, the liquefied natural gas is transferred from tanks 2to the vaporizer 4, where the fluid is regasified before beingtransferred through the buoy structure 7 and via one of the submergedpipelines 9 and 10 for bringing natural gas ashore to the pipelinesystem 12.

[0042]FIG. 3 illustrates the principle for the organization of theconnection of the various vessels to the buoy structures as well astransport to and from the loading station where LNG is loaded on boardthe vessel. In this example four vessels are included in the system. Asrepresented by the legends in FIG. 3, trips by the vessels can besynchronized so that unloading of regasified gas can take place almostcontinuously.

[0043] In FIG. 4 two buoy structures 7 are connected to the vessels 1and 1′ simultaneously. In the situation illustrated in FIG. 4, thevessel 1 is in the process of completing the unloading of natural gas,while the vessel 1′ has just received a buoy structure 7 in its recess5. By means of this arrangement a uniform transition will be achieved inthe unloading process from vessel 1 to vessel 1′, and a continuous flowof natural gas into the submerged pipeline system will thereby bemaintained.

[0044]FIG. 5 is a simplified schematic view of the second embodiment ofa retrofitted LNG carrier “Shuttle and Regas Vessel (RSV)” according tothe invention. The vessel 1 is illustrated provided with a plurality oftanks 2 for storing liquefied natural gas, one or more vaporizers 4 andthrusters 6, as in the embodiment of FIG. 1. At the vessel's bow portionthere is provided a projecting structure 15 which forms a receivingarrangement for a transfer structure 17 that is connected to a riser 8.Projecting structure 15 and transfer structure 17 can be constructed,coupled and operated according to principles already known in the art. Astructure 15′ ensures connection between vaporizer or vaporizers 4 andprojecting structure 15. As a substitute for traditional mooring linesthe vessel is provided with a dynamic positioning system 4′.

[0045] Transfer structure 17 is secured by a line 19 to a buoy 21 thatfloats on the water surface 23. When transfer structure 17 is not inuse, it will be submerged. Line 19 is used to retrieve transferstructure 17 for connection to projecting structure 15.

[0046] It is readily apparent that the a plurality of vessels as shownin FIG. 5 can be connected to a pipeline system in the manner shown inFIGS. 2 and 4 and can be organized to travel in the manner shown in FIG.3.

[0047] While the description above refers to particular embodiments ofthe present invention, it will be understood that many modifications maybe made without departing from the spirit thereof. The accompanyingclaims are intended to cover such modifications as would fall within thetrue scope and spirit of the present invention.

[0048] The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims, ratherthan the foregoing description, and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

What is claimed is:
 1. A system for delivering a fluid to a shore-basedfacility, said system comprising at least one vessel, at least onecoupling structure and a pipeline connected between said couplingstructure and the shore-based facility, wherein said vessel is operatedto be periodically connected to said coupling structure for transferringthe fluid from the vessel through said coupling structure and saidpipeline directly to the shore-based facility for further distributionof the fluid.
 2. The system according to claim 1, wherein at least twovessels and at least two coupling structures are included in the systemand the vessels travel in continuous shuttle traffic between a loadingstation and the coupling structures and at least one vessel isperiodically connected to one of the coupling structures fortransferring the fluid from the vessel through the coupling structure.3. The system according to claim 1, wherein the vessel is a retrofittedLNG tanker.
 4. The system according to claim 1, wherein the fluid isnatural gas.
 5. The system according to claim 1, wherein the vessel isequipped with a vaporizer.
 6. A system for unloading a fluid from avessel, the vessel having a hull provided with a downwardly facingrecess, said system comprising a submerged buoy structure and a pipelinesystem connected to said buoy structure for transferring fluid fromstorage in the vessel to a shore-based infrastructure, wherein said buoystructure is constructed to be inserted into said recess for connectingsaid pipeline system to the storage in the vessel.
 7. A system forunloading a fluid from a vessel, comprising; a transfer structure; apipeline system connected to said transfer structure; and a receivingstructure that projects from the vessel's bow area for receiving andcoupling to said transfer structure for transferring fluid from storagein the vessel to a shore-based infrastructure via said receivingstructure and said transfer structure.
 8. A system for unloading from avessel provided with storage tanks for storing natural gas in a liquidstate (LNG), wherein the system comprises a vaporizer provided on thevessel for vaporizing the liquefied gas into a gaseous state on arrivalat an unloading point, a buoy structure insertable into a bottom area ofthe vessel's hull for transferring gas in the gaseous state from thevessel, and a transport system including a submerged pipeline fortransferring the natural gas from the buoy structure to a shore-basedinfrastructure.
 9. A system for unloading from a vessel provided withstorage tanks for storing natural gas in a liquid state (LNG), whereinthe system comprises: a vaporizer provided on the vessel for vaporizingthe liquefied gas into a gaseous state on arrival at an unloading point,a transfer structure for transferring gas in the gaseous state from thevessel, the transfer structure being connected to a receiving structureprojecting from the vessel's bow area, and a transport system includinga submerged pipeline for transferring the natural gas from the buoystructure to a shore-based infrastructure.
 10. A vessel having a hull,said vessel comprising: storage tanks for storing natural gas in itsliquid state, a vaporizer for regasifying the liquefied natural gas onarrival at an unloading point, in combination with one of: a buoyreceiving recess arranged in the bottom area of the vessel's hull forconnection to a buoy structure; and a receiving structure projectingfrom the vessel's bow area for connection to a transfer structure, fortransferring natural gas from the vessel to a submerged pipeline system.11. A method for transferring a fluid to a shore-based facility,comprising: providing at least one vessel having fluid storage meanscontaining the fluid and equipment to receive a coupling structure;navigating the vessel to a location proximate to the shore-basedfacility; at the location proximate to the shore-based facility,connecting a coupling structure that is coupled to a pipeline to theequipment and coupling the fluid storage means to the pipeline via thecoupling structure; and transferring the fluid via the couplingstructure and the pipeline to the shore-based facility.
 12. The methodof claim 11 further comprising, before said step of transferring,gasifying the fluid.
 13. The method of claim 12, wherein: the equipmenton each vessel includes a downwardly facing recess in the vessel hullcommunicating with the fluid storage means and the coupling structure isa buoy structure that is connected to the pipeline; said step ofproviding at least one vessel comprises providing a plurality of thevessels; said step of navigating comprises navigating each vessel insuccession to the location proximate to the shore-based facility; andsaid step of connecting a coupling structure comprises introducing thebuoy structure into the recess of each vessel.
 14. The method of claim13, wherein said step of introducing a buoy comprises introducing adifferent buoy into the recess of each vessel.
 15. The method of claim14, wherein said step of transferring is carried out from each vessel insequence.
 16. The method of claim 12, wherein: the equipment on eachvessel includes a receiving structure projecting from the vessel andcoupled to the fluid storage means and the coupling structure is atransfer structure that is coupled to the pipeline; said step ofproviding at least one vessel comprises providing a plurality of thevessels; said step of navigating comprises navigating each vessel insuccession to the location proximate to the shore-based facility; andsaid step of connecting a coupling structure comprises connecting thetransfer structure to the receiving structure.
 17. The method of claim16, wherein said step of connecting a transfer structure comprisesconnecting a different transfer structure to the receiving structure ofeach vessel.
 18. The method of claim 17, wherein said step oftransferring is carried out from each vessel in sequence.